"There's a lot of reference material, so we came to know these hallways through these documentaries, so the first time I walked onto the finished set, it was like you walked into the documentary," she said. "And it's haunted."

The cast agreed with Stewart that the Whittier prison had a strange vibe. "Walls have memories," she added.

Stewart, who has also been to Sundance with several other films over the years, shared an interesting story about meeting founder Robert Redford at the fest.

"He was, like, 'How you doing?' And I shook his hand, and he was, like, 'Are you nervous?' and I was, like, 'Yeah, I'm shitting my pants,' and he was, like, 'Good, it would be weird if you weren't,'" she said.